Why is this an issue?

When a loop is filtering, selecting or aggregating, those functions can be handled with a clearer, more concise LINQ expression instead.

Noncompliant code example

var result = new List<string>();
foreach (var element in collection)  // Noncompliant
{
  if (condition(element))
  {
    result.Add(element);
  }
}

foreach (var element in collection2) // Noncompliant
{
  var someValue = element.Property;
  if (someValue != null)
  {
    result.Add(someValue);
  }
}

Compliant solution

var result = new List<string>();

foreach (var element in collection.Where(x => condition(x)))
{
  result.Add(element);
}

foreach (var someValue in collection2.Select(x => x.Property).Where(y => y != null))
{
  result.Add(someValue);
}